Router AR4640 fails in establishing GRE tunnel

Publication Date:  2012-07-27 Views:  201 Downloads:  0
Issue Description
As router AR4640 acts as GRE, the tunnel interface configures IP addressthe peer E company does not configure addresses at tunnel interface, so the tunnel over two ends cannot be established.

Alarm Information
No

Handling Process
As router AR46 acts as GRE, interface tunnel must configure IP addresses. Change the configuration of company E devicesconfigure the Ip addresses of interface TUNNELthe GRE tunnel over two ends can be established.

Root Cause
As router AR4640 acts as GRE, the tunnel interface must configure IP addressthis address is for the use of generating tunnel. Some devices do not configure addresses, because these addresses encapsulate GRE via the default address or borrowing other alien addresses.

The following processes are to encapsulate GRE
firstly, after the interface connecting with the interior network receives the IP packets, it will deliver it to the Ip protocol to do routing arrangement. The IP protocol checks the destination address domain in the IP packet header to ascertain how to route the packet. If the destination address of the packet is found to travel out from the tunnel interface Tunnel X, GRE encapsulation will be enabled. According to the source destination address defined by the interface Tunnel X
encapsulate the original packets. The destination address of the IP packet after completing GRE encapsulation (the protocol type is 7is the defined destination address by Tunnel X(generally, it is some interface address of the remote router),the source address is defined by Tunnel Xgenerally it is some interface address on the local router),the Ip packets after encapsulation will deliver it to the Ip module to processaccording to the corresponding destination address and the routing entries to determine to travel through which interface, and deliver it to the corresponding network interface to arrange.
the following is one receiving and sending flow of one Ping packet via GREtunnel. The first step is to establish routes via tunnel interface. Tunnel interface has no the IP addresses, it will not establish layer 3 routes, because the next hop of the route must have layer 3 IP addresses. If configure the static routes, the two ends of tunnel interface can belong to the various network segments; but if we want to run RIP/OSPF etc dynamic routing protocols, the two ends of the interface addresses of tunnel must belong to the same network segment.
00:07:31: IP: s=192.168.2.1 (local), d=192.168.1.1 (Tunnel0), len 100, sending
00:07:31:     ICMP type=8, code=0
00:07:31: IP: s=1.1.1.2 (local), d=1.1.1.1 (Ethernet0/0), len 128, sending, prot
o=47
00:07:31: IP: s=1.1.1.1 (Ethernet0/0), d=1.1.1.2 (Ethernet0/0), len 128, rcvd 3,
proto=47
00:07:31: IP: s=192.168.1.1 (Tunnel0), d=192.168.2.1, len 100, rcvd 4
00:07:31:     ICMP type=0, code=0
 

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